1,721,139 research outputs found
Carbonization Study of Cellulose Nanocrystals and Super Engineering Plastic Based Nano Composite Fibers
Full text linkDepartment of Materials Science and EngineeringCellulose Nanocrystals (CNC) have been regarded as a versatile precursor for carbon nanomaterials. CNC can be converted into carbon materials by hydrothermal treatment and subsequent carbonization process. Due to high crystallinity and structural regularity of CNC, carbonized CNC would give well-ordered graphitic structure compared to other cellulose-based carbon materials. In chapter 2, carbonization study of CNC covers the effect of heat treatment conditions on the structural development mechanism of CNC over the range of carbonization temperature from 1000 to 2500 oC. We have conducted experiments to study the effect of oxidative stabilization process on the structural development of CNC-based graphite. Compared to the carbonization mechanism of pristine CNC, stabilized CNC was prepared by heat treatment at 250 oC for 1hr. In addition, the resultant graphitic structure of carbonized cellulose nanocrystals was systemically analyzed by transmission electron microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. TEM data clarified that carbonized CNC prepared from stabilized samples (S-cCNC) gave rise to more highly ordered graphitic structure with little distortion site and defect points compare to D-cCNC over the whole temperature range of carbonization. Structural development mechanisms of both C- and S-cCNCs were systematically traced by Raman spectroscopy. Peak fitting results of Raman spectra evidenced structural conversion from disordered carbon to the graphitic structure.
In chapter 3, PES/CNC composite fibers were prepared by dry-jet wet spinning and their morphology and tensile properties were characterized. CNC with high Young???s modulus, crystallinity and aspect ratio can be regarded as a nano-size reinforcing agent. Dispersion of CNC was investigated by Dynamic Light Scattering (DLS) and Scanning Electron Microscopy (SEM). Upon using bath-type sonication with a power of 20 J/s, 48 hr sonication time was required to obtain well-dispersed CNC phase in N,N-Dimethylacetamide (DMAc). Experimental results showed that the tensile modulus of PES/CNC1 composite fibers were 4.7 GPa about 17% higher than control PES fibers.clos
?????????????????? ??????????????? ????????? ??? ????????? ???????????? ?????? ??????
No full textDepartment of Materials Science and Engineeringclos
Effect of Material Design on the Properties of Organic & Inorganic Solutions
No full textDepartment of Materials Science and Engineeringclos
Study of Polyacrylonitrile/Cellulose Nanocrystal Composite Fibers via In-situ Polymerization, and Super Engineering Plastic Based Nano Composite Fibers
Full text linkDepartment of Materials Science and EngineeringThe polymer nanocomposite is composed of a nanoscale reinforcing filler inside a polymer matrix. There are differences in the properties of composite materials depending on the shape and size of the filler. As the size of the filler decreases, the surface area-to-volume ratio increases and the interaction between the nanoparticles and the polymer matrix increases, and thus the enhanced properties of the composite also can be obtained. Cellulose nanocrystal (CNC) is a crystalline nano material after removing amorphous segments from cellulose. CNC is widely studied to be used as nano reinforcing fillers of composite materials due to its one-dimensional shape, high mechanical properties, many functional groups, high surface area, and eco-friendly characteristics. Based on these features, the polymer nanocomposite fibers containing CNC have been mainly studied.
In chapter 2, polyacrylonitrile (PAN) was synthesized by in-situ polymerization method in the solvent where the cellulose nanocrystal (CNC) was dispersed. Then, the efficiency of the spinning process is increased by using a polymerization solution that has not undergone additional purification directly as a spinning dope. As-spun fiber was fabricated using dry-jet wet spinning method, and in-situ PAN/CNC fiber having TDR 15, 16.5, 18, 19.5 was manufactured through a post-drawing process. The mechanically stirred ex-situ PAN/CNC fiber and control PAN fibers were prepared and compared with the behavior of mechanical properties of the in-situ PAN/CNC fibers. In the case of as-spun, TDR 15, 16.5, 18 fibers, in-situ PAN/CNC fibers had the 0.96 GPa of tensile strength and 19.8 GPa of tensile modulus which are the highest values. This is because the interaction between the PAN chain and CNC inside the in-situ PAN/CNC fiber was the greatest, and this maximized the reinforcing effect of the CNC on composite fibers. Hereafter, three types of PAN precursor fibers (control PAN, in-situ PAN/CNC, and ex-situ PAN/CNC at TDR 18) were stabilized at 260 ??? for 3 hours and carbonized at 1300 ??? and 1400 ??? to produce carbon fibers. Carbon fibers using in-situ PAN/CNC as a precursor showed the outstanding tensile properties. By analyzing the structural evolutions through the Raman spectra, it was confirmed that the IG/ID ratio and degree of decreasing amorphous part of carbonized in-situ PAN/CNC fibers were the largest. It means that the efficiency of forming a graphitic structure is changed by the low molecular weight material contained in the in-situ PAN/CNC precursor fibers. This research suggested that the CNC and low molecular weight polymers remained inside the fibers after direct spinning affect the mechanical properties of fibers.
In chapter 3, Poly(Arylene Ether Sulfone)(PAES)/cellulose nanocrystal (CNC) nanocomposite fibers were fabricated using dry-jet wet spinning and post-drawing method at CNC concentrations of 0, 1, 5, 10 wt% with respect to the polymer. TDR of control PAES and PAES/CNC nanocomposite fibers were 4.8, 5.76, and 6.4, respectively. The tensile strength and modulus of control PAES fibers were 122.0 MPa and 3.2 GPa, respectively, at TDR 6.4. The tensile strength of PAES/CNC1 at TDR 6.4 is 170.8 MPa which is the highest value and the highest tensile modulus value is 6.1 GPa of PAES/CNC10 at TDR 6.4. The tensile modulus continuously increased according to the CNC content and draw ratio. This is due to the increase in CNC alignment and the high modulus of the CNC itself. However, the tensile strength increases with the draw ratio, but tends to decrease as the CNC content increases from 1 wt% to 10 wt%. From the above trend, it was found that CNC acts as an element that inhibits the alignment of the PAES chains. This study suggested that the maximization of mechanical properties of amorphous polymer nanocomposite fibers can be achieved by optimizing the CNC content and fiber manufacturing process.clos
????????? ???????????????????????? ??????????????? ????????? ????????? ????????? ???????????? ?????? ????????????
No full textDepartment of Materials Science and EngineeringImprovements in the properties of materials are constantly demanded with increasing industrial progress. As a result, much research involves the development of new materials that have superior strength and functional properties. One of the methods that has attracted attention for a long time is the development of materials using the concept of composites. More recently, nanocomposites, composed of a nanofiller and a matrix material, have attracted substantial interest. Nanocomposites have the advantage of producing a considerable reinforcing effect by interphase formation in addition to benefiting from the properties of the filler itself. Even a small proportion of a nanofiller can greatly amplify desired propertiesthis is because of the high specific surface area of the nanofiller and physical/chemical interactions between the nanofiller and matrix. To produce high-performance nanocomposites, the degree of interphase formation with respect to various factors of nanofillers (e.g., surface functionality, size, dimension, degree of dispersion) should be understood first and then maximized. Subsequently, the matrix behavior in the interphase should also be considered.
Nanocarbon materials that can be used as nanofillers have been actively applied in various fields such as energy, purification, biotechnology, and composites. However, the most studied nanocarbons, such as carbon nanotubes or graphene, entail extensive processing steps for mass production. Further, there are limitations in that post-treatment processes, such as catalyst removal, are necessary to ensure high purity.
In this dissertation, interphase formation and behavior in various nanocomposite systems and the correlation between the processing, structure, and properties of nanocomposites are elucidated in Chapters 2???5. The effect of interfacial interactions on the conformational variations in the interphase and the resulting properties of polymer-matrix nanocomposites are presented in Chapters 2???3. The degree of templating and confinement effects of various carbon nanofillers on the structural development in carbon-matrix nanocomposites are discussed in Chapter 4 with respect to the dimension, quantity, and dispersion state of the nanofillers. In addition, the effect of nanofillers in nanocomposites is discussed based on the carbon fiber manufacturing process, which is a process that entails changes from the polymer matrix to the carbon matrix, in Chapter 5. Nanocarbon materials are produced through a simple carbonization process from novel precursors, and an in-depth understanding of their morphological and structural evolution mechanism is provided in Chapters 6???7.
The key objectives of the current dissertation are to provide a fundamental understanding of the physical phenomena of materials by observing the matrix behavior in the interphase of nanocomposites and to elucidate the structural evolution mechanisms of novel precursors for nanocarbon.ope
????????????????????? ?????? ?????? ??? ??? ????????? ?????????????????? ???????????? ??????????????? ?????? ??????????????? ??????
No full textDepartment of Materials Science and Engineeringclos
???????????? ????????????????????? ?????? ?????? ??? ????????? ????????? ??????, ?????? ??????????????? ???????????? ?????? ?????? ??????
Full text linkDepartment of Materials Science and EngineeringCellulose nanocrystals (CNCs) are extracted from cellulose materials by removing amorphous parts through acid hydrolysis. The nanometric CNCs with rod shape have intrinsically well-ordered crystalline structures. CNCs have been used as nanofillers for nanocomposites and promising carbon precursor based on the high mechanical properties, natural abundancy, high crystallinity, high surface area, and many functional groups on the surface. Based on these features, the carbonized CNCs and the pristine CNCs have been mainly studied for the applications.
In chapter 2, spray-dried CNCs (SD-CNC) were carbonized to prepare the highly efficient carbon anode material for sodium ion batteries (SIBs). Development of SIBs paves a new way as an alternative of lithium-based batteries. Even though sodium has similar chemical properties to lithium, it is unable to achieve facile intercalation reaction of sodium ion in graphite because of the larger ionic radius of Na (102 pm) and narrow interlayer distance of graphite (0.335 nm). Thus, new carbon-based anode materials for SIBs should be developed. SD-CNCs were carbonized at the various carbonization temperature (i.e., 800???2500 ???) and the anodes for SIB were prepared from the carbonized SD-CNCs. The microstructure of carbonized SD-CNC was investigated and electrochemical performance was measured. Upon increasing carbonization temperature, the less ordered structures are decreased, resulting in the decrease of the irreversible and sloping capacities. The ordered crystal structure with d-spacing above 0.37 nm are developed up to 1500 ??C, resulting in the increase of plateau capacity. The d-spacing is suddenly dropped down to 0.345 nm at 2500 ??C, and it leads to the dramatical decrease of plateau region. By correlating the microstructure with the electrochemical performance, the sodium ion storage mechanism was derived. The sloping capacity (above 0.2 V) is attributed to the sodium ion adsorption to the structural defects and the plateau region (below 0.2 V) is attributed to the sodium ion intercalation between graphene layers. Among the carbonized SD-CNCs, the carbonized SD-CNC show the best performance in SIBs, which is the reversible capacity of 310.6 mAh g???1 at 10 mA g???1 with highest initial coulombic efficiency (ICE) of 85%. Also, it shows the excellent specific capacity retention of 92.3% even after 400 cycles at 100 mA g???1.
In chapter 3, freeze-dried CNCs (FD-CNCs) with nanometric and fibrillar morphology were prepared by freeze drying for the precursor of carbon nanofillers from CNCs with the well-ordered crystal structure. Then, FD-CNCs are carbonized for the preparation of high-surface area carbons. Structural evolution mechanism of FD-CNCs was investigated during carbonization from 1000 to 2500 ??C. The weak and amorphous structure from the pristine state was converted into amorphous carbon or amorphous (A)-component with heat-treatment while the intrinsically highly crystallized parts become turbostratic (T)- or graphitic (G)-components at the low carbonization temperature of 1000 ??C. The FD-CNCs undergo four stages of structural development with carbonization temperature: In stage 1, the carbons with turbostratic structure are developed. In stage 2, small carbon crystallites with many defects are observed while the more ordered structure is seen as compared to the stage 1. In stage 3, graphitic crystals begin to develop, and subsequently in stage 4, the size of graphitic crystals dramatically increases via coalescence and lateral inter-fusion between crystals with removal of the defects. Meanwhile, the effects of oxidative stabilization are negligible due to the rapid heat diffusion and uniform structural change based on the high surface area of FD-CNCs. Carbonized FD-CNCs show the superior dispersibility due to the high surface area, indicating the possibility for carbon nanofillers.
In chapter 4, CNCs were applied as a reinforcement to enhance the mechanical properties of polymer. Polyether imide (PEI)/cellulose nanocrystal (CNC) nanocomposite fibers were dry-jet wet spun at CNC concentrations of 0, 1, 3, and 5 wt.% with respect to the polymer. The as-spun fibers were drawn in the draw ratio (DR) range from 2.8 to 7, and the behavior of their mechanical properties and fiber structures has been studied upon drawing at various amounts of CNC. At a DR of 7, the tensile strength and modulus of control fibers were 466.2 MPa and 8.1 GPa, respectively, and those of PEI/CNC3 fibers were 408.9 MPa and 9.2 GPa, which are the highest of any mechanical properties reported previously. The increase in tensile modulus by the addition of CNC was significantly influenced by the alignment of CNC along the fiber axis and the high specific modulus of CNC itself, rather than by the alignment of PEI polymeric chains. CNC, which is a crystalline material, already showed a high degree of orientation in the as-spun fibers and they became further aligned rapidly with drawing. Whereas, the orientation of amorphous polymeric chains developed slowly and exhibited still a low orientation factor even after drawing to a maximum extent. Further, the degree of orientation of PEI chains in the composite fibers was lower than that in the control fibers, indicating that the CNC disturbed PEI alignment to the axial direction. This research suggested that optimizing processing condition can maximize the mechanical properties of even amorphous polymers and adding nanofillers can further improve the properties.clos
??????????????? ????????? ???????????? ??????
No full textDepartment of Materials Science and Engineeringclos
Correlation between the Microstructure and the Mechanical Properties of PAN-based Stabilized/Carbonized Fibers
Full text linkDepartment of Materials Science and EngineeringPolyacrylonitrile (PAN) have been regarded as a versatile precursor for high strength carbon fiber. Carbon fiber has been developed 50 years ago, however microstructure developed mechanism and the correlation between microstructure of carbon fiber and mechanical properties has not been clearly established. Using continuous heat-treatment equipment in UNIST, the microstructure of the PAN precursor fiber was defined. In chapter 2, we found optimization conditions on manufacturing process for high strength carbon fiber. The carbonization temperature is the key parameter than another parameter. To determine effect of heat-treatment process, the sample was collected for each temperature zone.
In the chapter 3, the cyclization reaction defined in different stage. From 200oC, the cyclization reaction occurred which effect to loss of tensile strength. At 240oC, the crystallite of PAN structure transition to ladder structure. This crystal destroyed effected to the modulus of stabilized. In the fracture morphology image, surface show the tough region. Therefore, the core consist more amorphous PAN structure reacted first, then the crystal phase reacts.
In chapter 4, the stabilized fibers were prepared from the chapter 3. As the carbonization temperature increase, the density and the mechanical properties were changed. We defined the four stage of carbonization. At the stage 1, from 400 to 580oC, the residual PAN structure cyclized in the fiber. At the stage 2, from 580 to 900oC, the intermolecular crosslinking by dehydrogenation occurs. At the stage 3, from 900 to 1200oC, the intermolecular crosslinking by denitrogenation with high weight loss in the carbonized fiber. At the stage 4, the structure of graphitic structure growth. The mechanical properties highly increase in carbonization region, stage 3 and 4. With Raman analysis, microstructure of the surface-skin-core region detected. In the stage 2, the high decrease of D-band in core due to the combination of high excess heat. In the stage 3, the amorphous carbon developed in core region due to the shrinkage process.
This thesis about tracing of evolution in the PAN fiber and give the key information to manufacturing the carbon fiber. By optimized the carbonization process, the manufacturing cost and time will reduce and might be reduce the cost of final carbon fiber. By high performance carbon fiber in low cost, the application of carbon fiber expands in the industry.clos
- …
